It is well-known to use oil filters to decontaminate oil that contains suspended impurities. A major application is the use of such filters for filtration of machinery oil such as hydraulic oil and the oil used in the lubrication system of engines, for example in gears and bearings. Efficient filtration of oil in any oil-filled system is essential for enhancing longevity and performance.
DE 195 11 450 (Münkel) discloses a bypass-mounted filter assembly for filtering fluid from a hydraulic-fluid reservoir in a bypass line, the filter having a motor-driven pump and a rigidly mounted housing which is closed off by a cover and which contains a replaceable filter element. The filter housing is connected directly to the pump, and forms a vessel designed to hold the clean fluid when the filter element is changed in order to make it possible to replace the filter element or renew the filter simply without any danger of contaminating the clean oil downstream of the filter. In an embodiment, a differential pressure gauge monitors the pressure difference between the inlet and outlet sides of the filter. In case a specified difference pressure value is exceeded, an electric signal can be used to stop the motor of the pump. A check valve prevents oil from running back from a higher located tank for the filtrated oil. DE 195 11 450 does not disclose or suggest how to solve filtration problems in case of an oil having a content of air, which may impair the filtration.
DE 199 33 620 (Locher) discloses an oil filter for hydraulically operated machines with rotating parts, especially automatic gear boxes of vehicles, having a membrane inserted in it to remove air bubbles present in the oil stream. A membrane area of one dm2 removes about 20 ml/h air. Such low capacity makes this solution unsuitable for use in large scale.
EP1424116 (Hesterwerth et al.) discloses an air bubble separator with an oil collecting chamber for degassing oil through an air volume provided over the oil. Retaining plates arranged behind each other in the flow direction are provided in an inlet region behind an oil inlet. A flow channel with alternating wide and narrow free cross-sectional dimensions is provided in the flow direction behind the retaining plates. Such separator is unsuitable in case of very small air bubbles.
WO 99/35435 (Salavamäki et al.) discloses a method for bringing the content of air in lubrication oil or hydraulic oil into a state of equilibrium in during circulation. By this method, air bubbles of an order of magnitude of 1.5 mm or larger rise to the oil surface in the oil tank, whereas smaller bubbles are dissolved under the hydraulic pressure at the bottom of the sufficiently high level of oil in the tank. This pressure is in the order of 0.5 bar (0.05 MPa). FIG. 4 of WO 99/35435 shows a filter for cleaning the oil taken pumped from the bottom of the oil tank. After the filter is a cooler. However, this paper is silent about the particular filtration conditions.
Today's machinery must be fast, accurate and economical. This increases the requirements to machinery oils such as hydraulic oil and lubrication oil, as the machinery tolerances become finer, such as about 4 μm and often below 3 μm. Since particles of this size can cause long-term wear, reducing their number to as few as possible is a priority.
As an example, the gearbox of a wind turbine is lubricated with relatively high viscous oil (such as an average viscosity at 40° C. of 320 mm2/s; ISO VG 320 according to DIN 51 519). This oil can be cleaned using a 3 μm filter insert. By one passage through such filter, the removal of present particles larger than 4 μm by laboratory experiments is typically a drop of 3 to 4 ISO 4406 classes (for example from range number 19 to 16 or 15, see table 1 below). However, in practice the efficiency is often found to be drastically lower. This means that several passages through the filter are needed to obtain acceptable oil cleanliness. This is very costly. Accordingly, there is still a demand for a more efficient method for filtration of contaminated oil.
The present invention is based on the consideration of the above-mentioned discrepancy between the results in the laboratory and oil filtration in practice. In this connection, it was decided to analyse possible problems caused by incorporation of air in the oil during working. A problem with such analysis is to obtain representative oil samples when the oil is not only contaminated with suspended solid particles, but also contains both dissolved air and also air bubbles. When collecting such samples, the air bubbles may fuse together to larger bubbles and rise to the surface of the oil on standing.
As mentioned above, the gearbox in a wind turbine is lubricated with an oil having a relatively high viscosity. It was found that when the wind turbine is working, air is incorporated in the oil in amounts of typically 5 to 20 vol/vol %. These amounts of air are present as smaller and larger bubbles of free air. Typically, the oil also contains 8 to 12 vol/vol % air dissolved in the oil at atmospheric pressure. These amounts are not included in the 5 to 20 vol/vol % of air bubbles. It was further found that the incorporated air bubbles in the gearbox are agitated to form very fine bubbles giving turbid oil, which appears as froth. The suspended air bubbles may have a diameter down to 1 μm.
Based on these findings, experimental air-containing oil was prepared as a froth using a small pump mixing air into the oil. Using this air-containing oil froth, the filtration results were more in agreement with those found in practice, i.e. with a similar bad efficiency.
Having thus realised that the filtration problems were due to the content of air bubbles in the oil, one might conclude that the air bubbles had to be removed before the filtration using one of the above-mentioned complicated, non-efficient and expensive methods for such removal.
However, especially when the air bubbles are very fine, as the case is for the froth of lubrication oil with very small air bubbles from the gearbox of wind turbines, such separation of air bubbles from high viscosity oil is very troublesome and time consuming.
Thus, there is a need for a less complicated solution.
The object of the present invention is to provide a method and a device for simple, but still efficient removal of solid contaminants from air-containing oil, especially lubrication oil or hydraulic oil.
In the present application, the term “air-containing oil” refers to oil with an amount of non-dissolved free air bubbles of at least 0.2 vol/vol % normally appearing as a froth.